1. Field of the Invention
The invention relates to a power transmission system comprising: right and left independent transaxles for driving respective right and left axles; and a working power clutch unit incorporating a working power clutch. The invention also relates to a working vehicle equipped with the power transmission system.
2. Related Art
Conventionally, there is a well-known working vehicle equipped with right and left independent transaxles for driving respective right and left axles, such as pedestrian-controlled lawn mowers as disclosed in U.S. Pat. No. 5,127,215A (reference 1) and WO 97/15764 (reference 2). This type working vehicle is advantageous in its compactness, its large space for a working device, and its ability of sharp turning (i.e., zero-turning).
In each of the vehicles disclosed in the references 1 and 2, an engine serving as a prime mover is disposed so as to extend its output shaft vertically downward. Two pulleys are provided on the engine output shaft. One pulley is connected to input pulleys of respective right and left transaxles through a belt. The other pulley is connected to an input pulley of a working device (a mower unit) through another belt. In this way, the engine power is distributed between the working device and the pair of right and left transaxles for traveling of the vehicle.
As shown in the references 1 and 2, in this type working vehicle, the pair of transaxles are covered over with a frame, and the engine is mounted on a horizontal upper flat surface of the frame.
With respect to the clutch for the working device, as disclosed in the reference 2, for example, a clutch brake is interposed in the pulley on the engine output shaft for transmitting power to the mower deck.
Each of the right and left transaxles is an integral hydrostatic transaxle (hereinafter referred to as “IHT”) having a transaxle housing incorporating mutually fluidly connected hydraulic pump and motor constituting a hydrostatic transmission (hereinafter referred to as “HST”). The hydraulic pump includes a vertical pump shaft, on which the input pulley is disposed above the transaxle housing so as to receive the power from the engine. Further, in the IHT shown in the reference 2, the hydraulic motor includes a vertical motor shaft advantageously for lateral and fore-and-aft minimization. In this IHT, the vertical motor shaft projects at its top end upward from the housing, so as to have a dry brake between the top of the motor shaft and the top surface of the housing. Both of the brakes provided on the respective right and left IHTs are applied simultaneously for stopping the vehicle. One of the brakes is applied for zero-turning of the vehicle to the side corresponding to the applied brake.
Each of the above-mentioned hydraulically driven working vehicles requires a sufficient space below the frame for arrangement of the working device and the right and left transaxles. Further, if an electromagnetic clutch is used for switching on and off the power transmission to the working device, the space below the frame has to be larger for arrangement of the electromagnetic clutch. On the other hand, especially if the working vehicle having the power transmission system is made for riding, a heavy engine is desired to be lowered as much as possible so as to ensure safety in zero-turn.
It is important for lowering the engine to ensure a sufficient space below the frame for arranging the left and right transaxles, the working device and the electromagnetic clutch for the working device. In the arrangement as disclosed in the above references 1 and 2, the engine is disposed on the upper surface of the horizontal plate-shaped portion of the frame, and the transaxles are disposed below the horizontal plate-shaped portion of the frame. To lower the engine in this situation, the transaxles also have to be lowered. The lowered transaxles may interfere with the working device or the working power transmission mechanism (including the electromagnetic clutch). Conclusively, while the engine is desired to be lowered as much as possible for stability of the traveling vehicle, the transaxles have to be disposed at a considerably high position with respect to the working device.
If each of the left and right IHTs of the power transmission system has a vertical motor shaft, the above-mentioned dry brake is exposed above the housing. Such an exposed brake is poor in protection and is worn early. This problem is cleared if an alternative wet brake is disposed in a housing of the IHT. However, the question is where the wet brake is disposed in the IHT used for the power transmission system, while the IHT has the restricted vertical length and the vertical motor shaft.